
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">


<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
    
    <title>FaultMaps and implicit checks &mdash; LLVM 3.7 documentation</title>
    
    <link rel="stylesheet" href="_static/llvm-theme.css" type="text/css" />
    <link rel="stylesheet" href="_static/pygments.css" type="text/css" />
    
    <script type="text/javascript">
      var DOCUMENTATION_OPTIONS = {
        URL_ROOT:    './',
        VERSION:     '3.7',
        COLLAPSE_INDEX: false,
        FILE_SUFFIX: '.html',
        HAS_SOURCE:  true
      };
    </script>
    <script type="text/javascript" src="_static/jquery.js"></script>
    <script type="text/javascript" src="_static/underscore.js"></script>
    <script type="text/javascript" src="_static/doctools.js"></script>
    <link rel="top" title="LLVM 3.7 documentation" href="index.html" />
    <link rel="next" title="LLVM Developer Policy" href="DeveloperPolicy.html" />
    <link rel="prev" title="Bitsets" href="BitSets.html" />
<style type="text/css">
  table.right { float: right; margin-left: 20px; }
  table.right td { border: 1px solid #ccc; }
</style>

  </head>
  <body role="document">
<div class="logo">
  <a href="index.html">
    <img src="_static/logo.png"
         alt="LLVM Logo" width="250" height="88"/></a>
</div>

    <div class="related" role="navigation" aria-label="related navigation">
      <h3>Navigation</h3>
      <ul>
        <li class="right" style="margin-right: 10px">
          <a href="genindex.html" title="General Index"
             accesskey="I">index</a></li>
        <li class="right" >
          <a href="DeveloperPolicy.html" title="LLVM Developer Policy"
             accesskey="N">next</a> |</li>
        <li class="right" >
          <a href="BitSets.html" title="Bitsets"
             accesskey="P">previous</a> |</li>
  <li><a href="http://llvm.org/">LLVM Home</a>&nbsp;|&nbsp;</li>
  <li><a href="index.html">Documentation</a>&raquo;</li>
 
      </ul>
    </div>


    <div class="document">
      <div class="documentwrapper">
          <div class="body" role="main">
            
  <div class="section" id="faultmaps-and-implicit-checks">
<h1>FaultMaps and implicit checks<a class="headerlink" href="#faultmaps-and-implicit-checks" title="Permalink to this headline">¶</a></h1>
<div class="contents local topic" id="contents">
<ul class="simple">
<li><a class="reference internal" href="#motivation" id="id1">Motivation</a></li>
<li><a class="reference internal" href="#the-fault-map-section" id="id2">The Fault Map Section</a></li>
<li><a class="reference internal" href="#the-implicitnullchecks-pass" id="id3">The <code class="docutils literal"><span class="pre">ImplicitNullChecks</span></code> pass</a><ul>
<li><a class="reference internal" href="#make-implicit-metadata" id="id4"><code class="docutils literal"><span class="pre">make.implicit</span></code> metadata</a></li>
</ul>
</li>
</ul>
</div>
<div class="section" id="motivation">
<h2><a class="toc-backref" href="#id1">Motivation</a><a class="headerlink" href="#motivation" title="Permalink to this headline">¶</a></h2>
<p>Code generated by managed language runtimes tend to have checks that
are required for safety but never fail in practice.  In such cases, it
is profitable to make the non-failing case cheaper even if it makes
the failing case significantly more expensive.  This asymmetry can be
exploited by folding such safety checks into operations that can be
made to fault reliably if the check would have failed, and recovering
from such a fault by using a signal handler.</p>
<p>For example, Java requires null checks on objects before they are read
from or written to.  If the object is <code class="docutils literal"><span class="pre">null</span></code> then a
<code class="docutils literal"><span class="pre">NullPointerException</span></code> has to be thrown, interrupting normal
execution.  In practice, however, dereferencing a <code class="docutils literal"><span class="pre">null</span></code> pointer is
extremely rare in well-behaved Java programs, and typically the null
check can be folded into a nearby memory operation that operates on
the same memory location.</p>
</div>
<div class="section" id="the-fault-map-section">
<h2><a class="toc-backref" href="#id2">The Fault Map Section</a><a class="headerlink" href="#the-fault-map-section" title="Permalink to this headline">¶</a></h2>
<p>Information about implicit checks generated by LLVM are put in a
special &#8220;fault map&#8221; section.  On Darwin this section is named
<code class="docutils literal"><span class="pre">__llvm_faultmaps</span></code>.</p>
<p>The format of this section is</p>
<div class="highlight-none"><div class="highlight"><pre>Header {
  uint8  : Fault Map Version (current version is 1)
  uint8  : Reserved (expected to be 0)
  uint16 : Reserved (expected to be 0)
}
uint32 : NumFunctions
FunctionInfo[NumFunctions] {
  uint64 : FunctionAddress
  uint32 : NumFaultingPCs
  uint32 : Reserved (expected to be 0)
  FunctionFaultInfo[NumFaultingPCs] {
    uint32  : FaultKind = FaultMaps::FaultingLoad (only legal value currently)
    uint32  : FaultingPCOffset
    uint32  : HandlerPCOffset
  }
}
</pre></div>
</div>
</div>
<div class="section" id="the-implicitnullchecks-pass">
<h2><a class="toc-backref" href="#id3">The <code class="docutils literal"><span class="pre">ImplicitNullChecks</span></code> pass</a><a class="headerlink" href="#the-implicitnullchecks-pass" title="Permalink to this headline">¶</a></h2>
<p>The <code class="docutils literal"><span class="pre">ImplicitNullChecks</span></code> pass transforms explicit control flow for
checking if a pointer is <code class="docutils literal"><span class="pre">null</span></code>, like:</p>
<div class="highlight-llvm"><div class="highlight"><pre>  <span class="nv">%ptr</span> <span class="p">=</span> <span class="k">call</span> <span class="k">i32</span><span class="p">*</span> <span class="vg">@get_ptr</span><span class="p">()</span>
  <span class="nv">%ptr_is_null</span> <span class="p">=</span> <span class="k">icmp</span> <span class="k">i32</span><span class="p">*</span> <span class="nv">%ptr</span><span class="p">,</span> <span class="k">null</span>
  <span class="k">br</span> <span class="k">i1</span> <span class="nv">%ptr_is_null</span><span class="p">,</span> <span class="kt">label</span> <span class="nv">%is_null</span><span class="p">,</span> <span class="kt">label</span> <span class="nv">%not_null</span><span class="p">,</span> <span class="nv">!make.implicit</span> <span class="nv-Anonymous">!0</span>

<span class="nl">not_null:</span>
  <span class="nv">%t</span> <span class="p">=</span> <span class="k">load</span> <span class="k">i32</span><span class="p">,</span> <span class="k">i32</span><span class="p">*</span> <span class="nv">%ptr</span>
  <span class="k">br</span> <span class="kt">label</span> <span class="nv">%do_something_with_t</span>

<span class="nl">is_null:</span>
  <span class="k">call</span> <span class="kt">void</span> <span class="vg">@HFC</span><span class="p">()</span>
  <span class="k">unreachable</span>

<span class="nv-Anonymous">!0</span> <span class="p">=</span> <span class="p">!{}</span>
</pre></div>
</div>
<p>to control flow implicit in the instruction loading or storing through
the pointer being null checked:</p>
<div class="highlight-llvm"><div class="highlight"><pre>  <span class="nv">%ptr</span> <span class="p">=</span> <span class="k">call</span> <span class="k">i32</span><span class="p">*</span> <span class="vg">@get_ptr</span><span class="p">()</span>
  <span class="nv">%t</span> <span class="p">=</span> <span class="k">load</span> <span class="k">i32</span><span class="p">,</span> <span class="k">i32</span><span class="p">*</span> <span class="nv">%ptr</span>  <span class="c">;; handler-pc = label %is_null</span>
  <span class="k">br</span> <span class="kt">label</span> <span class="nv">%do_something_with_t</span>

<span class="nl">is_null:</span>
  <span class="k">call</span> <span class="kt">void</span> <span class="vg">@HFC</span><span class="p">()</span>
  <span class="k">unreachable</span>
</pre></div>
</div>
<p>This transform happens at the <code class="docutils literal"><span class="pre">MachineInstr</span></code> level, not the LLVM IR
level (so the above example is only representative, not literal).  The
<code class="docutils literal"><span class="pre">ImplicitNullChecks</span></code> pass runs during codegen, if
<code class="docutils literal"><span class="pre">-enable-implicit-null-checks</span></code> is passed to <code class="docutils literal"><span class="pre">llc</span></code>.</p>
<p>The <code class="docutils literal"><span class="pre">ImplicitNullChecks</span></code> pass adds entries to the
<code class="docutils literal"><span class="pre">__llvm_faultmaps</span></code> section described above as needed.</p>
<div class="section" id="make-implicit-metadata">
<h3><a class="toc-backref" href="#id4"><code class="docutils literal"><span class="pre">make.implicit</span></code> metadata</a><a class="headerlink" href="#make-implicit-metadata" title="Permalink to this headline">¶</a></h3>
<p>Making null checks implicit is an aggressive optimization, and it can
be a net performance pessimization if too many memory operations end
up faulting because of it.  A language runtime typically needs to
ensure that only a negligible number of implicit null checks actually
fault once the application has reached a steady state.  A standard way
of doing this is by healing failed implicit null checks into explicit
null checks via code patching or recompilation.  It follows that there
are two requirements an explicit null check needs to satisfy for it to
be profitable to convert it to an implicit null check:</p>
<blockquote>
<div><ol class="arabic simple">
<li>The case where the pointer is actually null (i.e. the &#8220;failing&#8221;
case) is extremely rare.</li>
<li>The failing path heals the implicit null check into an explicit
null check so that the application does not repeatedly page
fault.</li>
</ol>
</div></blockquote>
<p>The frontend is expected to mark branches that satisfy (1) and (2)
using a <code class="docutils literal"><span class="pre">!make.implicit</span></code> metadata node (the actual content of the
metadata node is ignored).  Only branches that are marked with
<code class="docutils literal"><span class="pre">!make.implicit</span></code> metadata are considered as candidates for
conversion into implicit null checks.</p>
<p>(Note that while we could deal with (1) using profiling data, dealing
with (2) requires some information not present in branch profiles.)</p>
</div>
</div>
</div>


          </div>
      </div>
      <div class="clearer"></div>
    </div>
    <div class="related" role="navigation" aria-label="related navigation">
      <h3>Navigation</h3>
      <ul>
        <li class="right" style="margin-right: 10px">
          <a href="genindex.html" title="General Index"
             >index</a></li>
        <li class="right" >
          <a href="DeveloperPolicy.html" title="LLVM Developer Policy"
             >next</a> |</li>
        <li class="right" >
          <a href="BitSets.html" title="Bitsets"
             >previous</a> |</li>
  <li><a href="http://llvm.org/">LLVM Home</a>&nbsp;|&nbsp;</li>
  <li><a href="index.html">Documentation</a>&raquo;</li>
 
      </ul>
    </div>
    <div class="footer" role="contentinfo">
        &copy; Copyright 2003-2015, LLVM Project.
      Last updated on 2015-09-08.
      Created using <a href="http://sphinx-doc.org/">Sphinx</a> 1.3.1.
    </div>
  </body>
</html>